The Ireland-headquartered drugmaker which is seeking to expand the market for its fish-oil pill, yesterday launched a legal case against the US Food and Drug Administration over alleged restrictions of its right to promote the drug for unapproved, or off-label, use.

The lawsuit, filed in federal court in New York, argues that the FDA's ban on promotions of their products for unapproved uses violates Amarin's right to free speech under the US Constitution's First Amendment.

Amarin is represented by eminent US constitutional lawyer Floyd Abrams.

It is the first lawsuit of its kind against the FDA since 2012, when a US court overturned the conviction of a salesman, Alfred Caronia, who had been talking to physicians about off-label uses of the narcolepsy drug Xyrem. It ruled that the First Amendment protected truthful and non-misleading off-label speech.

"Over the last 10 years the Supreme Court ... has provided more protection for commercial speech than had been the case before," said Mr Abrams, a partner at law firm Cahill Gordon & Reindel.

He said the court "has been more willing to conclude that commercial speech was protected, even though there were substantial arguments to the contrary".

Amarin sells Vascepa, a fish-oil pill approved by the FDA to treat patients with exceptionally high levels of blood fats known as triglycerides that have been linked to diabetes, kidney failure and pancreatic cancer.

Amarin wants to also market the drug for patients with high, but not exceptionally high, triglyceride levels.